This invention relates a laser diode protecting circuit in a laser diode drive having an automatic current control circuit (ACC circuit) for performing control in such a manner that laser diode current attains a set value. More particularly, the invention relates to a laser diode protecting circuit for protecting a laser diode by preventing an excessive emission from the laser diode when the laser diode is started up at low temperatures.
A deterioration in transmission characteristics due to wavelength fluctuation (chirping) cannot be ignored in high-speed optical communications. In addition, wavelength stability is extremely important in wavelength division multiplexing. For these reasons the laser diode drive is constructed by combining an ACC circuit and an ATC (Automatic Temperature Control) circuit and control is performed in such a manner that the laser diode current will attain a constant current value and the laser diode chip temperature (laser diode temperature) a constant temperature.
FIG. 15 is a block diagram illustrating an optical transmitter used in digital optical communication. The transmitter includes a laser diode drive 1 having a laser diode (LD) 1a, an ACC circuit 2 for performing control in such a manner that the laser diode current attains a set current value and an ATC circuit 3 for performing control in such a manner that the laser diode temperature attains a set value, optical fibers 4, 5, a D-type flip-flop (D-FF) 6 for passing a data signal DATA in response to a clock CLK, and a drive circuit (DRV) 7 for a light intensity modulator (IM) 8, which modulates light intensity in accordance with the "1", "0"logic of the data.
FIG. 16 shows an example of the ACC circuit 2. The laser diode (LD) is indicated at 1a. The ACC circuit 2 includes resistors R1.about.R3 having resistance values r.sub.1 .about.r.sub.3, respectively, a transistor TR1 and a comparator (Op Amp) IC1 constituted by an operational amplifier. The laser diode 1a, transistor TR1 and resistor R1 are serially connected. If id represents a current that flows through the laser diode 1a, then id.r.sub.1 will enter the inverting input terminal of the comparator IC1. On the other hand, a reference voltage V.sub.REF, obtained by voltage division by the resistors R2, R3, enters the non-inverting input terminal of the comparator IC1. The ACC circuit 2 brings the laser diode current id into line with the set current value by controlling the on/off operation of the transistor TR1 in such a manner that the terminal voltage id.r.sub.1 across the resistor R1 becomes equal to the reference voltage V.sub.REF. More specifically, the voltage V.sub.REF obtained by voltage division by the resistors R2, R3 becomes the voltage across the resistor R1 and a value obtained by dividing this voltage by the resistance value r.sub.1 becomes the current id that flows through the laser diode 1a. In other words, the base of the transistor TR1 is controlled by the comparator IC1 in such a manner that the resistor R1 will serve as a constant-current source the current value of which will be V.sub.REF /r.sub.1 at all times, thereby making it possible to obtain a constant current value even when the temperature varies.
FIG. 17 illustrates an example of the ATC circuit 3. The laser diode chip is shown at 1a. The ATC circuit includes a Peltier device 3a for heating or cooling the laser diode chip 1a depending upon the direction of the current, and a thermister 3b having a negative resistance characteristic for detecting the temperature of the laser diode chip 1a. The laser diode 1a, Peltier device 3a and thermister 3b are accommodated in a package 3c. The ATC circuit further includes resistors 3d, 3e, PNP, NPN transistors 3f, 3g and a comparator 3h. A voltage Vt (which conforms to the laser diode temperature) resulting from voltage division by the thermister 3b and resistor 3d is applied to the inverting input terminal of a comparator 3h, and a reference voltage V.sub.ref is applied to the non-inverting input terminal of the comparator 3h. The output terminal of the comparator is connected to the bases of transistors 3f, 3g. The emitter of the PNP transistor 3f is connected to V+, the emitter of the NPN transistor 3g is connected to V-, and the collectors of these transistors are connected to the Peltier device 3a.
When the laser diode chip is at a low temperature, the resistance of the thermister 3b increases, the voltage Vt decreases to establish the inequality Vt&lt;Vref and the output of the comparator 3h becomes positive. As a result, the transistor 3f is turned off and the transistor 3g is turned on so that a current flows in a direction that causes the heating of the Peltier device 3a, thereby heating the interior of the package 3c and raising the temperature of the laser diode. When the temperature of the laser diode chip rises, the resistance of the thermister 3b decreases and the voltage Vt increases to establish the inequality Vt&gt;Vref so that the output of the comparator 3g becomes negative. As a result, the transistor 3f is turned on and the transistor 3g is turned off so that a current flows in a direction that cools the Peltier device 3a, thereby lowering the temperature of the laser diode. The temperature of the laser diode is thus controlled so as to attain the set temperature.
When power is introduced to the optical transmitter of FIG. 15 at low temperatures to drive the laser diode 1a, the laser diode emits radiation excessively and the laser diode itself may be damaged. The reason for the excessive emission is as follows: The laser diode has a temperature characteristic of the kind shown in FIG. 18. It will be understood that the lower the temperature, the greater the power P needed to pass a constant laser diode current. If ACC stabilization time at which the laser diode current attains the set value by ACC is compared with stabilization at which the laser diode temperature attains the set value by ATC, it will be seen that ATC stabilization time is longer than ACC stabilization time. Consequently, when the laser diode is driven by introducing power at low temperature, the laser diode current attains the set value by ACC before the laser diode chip attains the fixed temperature owing to the delay involved in ATC, as a result of which the power of the emission from the laser diode increases and becomes so excessive as to degrade the characteristic of the laser diode and eventually destroy the same. In other words, though the laser diode current attains the target value owing to the ACC circuit, the laser diode temperature does not attain its target value. Accordingly, the laser diode produces an emission in excess of the target value.